buckling restrained braces tests eng

7/30/2019 Buckling Restrained Braces Tests Eng

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EUROSTEEL 2008, 3-5 September 2008, Graz, Austria

ONLY -STEEL BUCKLING-RESTRAINED BRACESAN EXPERIMENTAL STUDY

Mario DAniello

a

, Gaetano Della Corte

a

, Federico M. Mazzolani

a

a

University of Naples Federico II, Department of Structural Engineering, P.le Tecchio 80 80125 Naples, Italy

INTRODUCTION

Buckling-Restrained Braces (BRBs) are a relatively recent development in the field of seismic

resistant steel structures [1, 2, 3]. BRBs can be considered as one of the most efficient structural

system for resisting lateral forces due to earthquakes because (i) they provide complete truss action,

(ii) they exhibit a symmetric load-deformation behaviour (equal response in compression and

tension) and large energy absorption capacity. They are basically made of two components: (i) a

yielding steel core and (ii) an encasing member. The former component takes the axial force whilethe latter component restrains the brace from buckling in compression. In particular, it is possible to

provide this mechanical behaviour enclosing a ductile steel core (rectangular or cruciform plates,

circular rods, etc.) either in a continuous concrete filled tube or within a continuous steel tube. In

the first case, the brace is called unbonded BRB, because the surface between the core and the

sleeve is treated with unbonding materials to allow the relative displacement with the sleeve to be

developed. In the second case, the steel core is separated by the sleeve by a small gap and it is

usually called only-steel BRB. In both cases, the assembly is detailed so that the yielding core can

deform longitudinally independent from the mechanism that restrains lateral and local buckling. In

detail, only-steel BRBs have some advantages over unbonded braces. In fact, this type of BRBs

can be designed to be detachable. Hence, they could be inspected after each seismic event and, if

necessary, the yielded steel core could be replaced by a new one. Moreover, an only-steel BRB islighter than an unbonded one; this implies a technical and economical advantage during the

assembling. These considerations motivated the research presented in this paper that was addressed

to study a special only-steel detachable BRB, to be used for improving the seismic response of

existing buildings.

The examined BRB is a special detachable only-steel device, made of a rectangular steel plate

encased in a bolted restraining steel sleeve. In particular, the BRB systems were designed to be

hidden in the inner hole of facing walls of typical existing reinforced concrete (RC) structures. Full-

scale tests have been carried out showing the importance of the design and manufacturing of local

details, such as the gap between the core and the restraining unit, the contraction allowance, the

end-connection details. This paper shows the results of the last test, concerning a new typology of

only-steel BRB.

1 DESIGN ASPECTS OF THE TESTED BRBS

The basic concepts for the design of the tested devices descend from the experience matured within

the ILVA-IDEM project [4]. In that contest, two types of BRBs have been studied [4, 5]. The first

type (henceforth called type 1) was made using two rectangular tubes for the restraining unit. The

two tubes were fully welded together with steel plates. The second type (henceforth called type 2)

was detachable, being made again with two restraining rectangular tubes, but joined together by

means of bolted steel connections.

The BRB type under examination was derived from type 2, with some modifications. The new

tested BRB prototypes (henceforth called type 3 and type 4) have also been designed to bedetachable, but they differ in several aspects both from the previous one and among them. Both of

them essentially differ from their progenitor (type 2) in the restraining unit. In fact, instead of two

joined steel tubes, the restraining unit is constituted by two omega-shaped built-up sections, which


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structural frame members and infill walls. Lateral loads have been applied to the floors according to

a linear distribution. Both tests showed the formation of a weak story at the first floor. After the

tests, the structure has been partially repaired and the two BRB systems previously described have

been designed. In particular, the BRB systems were designed to be hidden in the inner hole of

facing walls as shown in Figure 2a where the braces (indicated with the dashed lines) are inserted

into the two perimetric bays per short building side. Figure 2b shows the brace configuration, with

the BRBs mounted only at the weak first floor. Moreover, in order to demonstrate the feasibility ofhiding the device into the space between the external claddings, the external facing wall has been

reconstructed in one bay (Figure 2b).

3 TEST RESULTS

Both tested systems showed a good overall response. However, a distinction must be made between

type 3 and type 4. In fact, the latter type showed the best performance with the larger deformation

capacity and overall ductility.

3.1 BRB type 3

In case of BRB type 3 the tested structure reached a maximum interstory drift of about 1.25%(Figure 3a), corresponding to the occurrence of an undesired local buckling phenomenon. Then, the

ductility was =b/y=1.25%/0.18%6.94 (Figure 3b). Figure 4 summarizes the damage pattern. Inparticular, Figure 4a shows the plastic tensile elongation of the brace. Figure 4b shows the collapse

of the external facing wall caused by buckling of the unrestrained end-portion of the BRB. Figures

4c and 4d show the unrestrained end portion of the brace in its final buckled configuration. Failure

of welds between the stiffener plates and the tapered core plate is also visible.

The reason for this unforeseen buckling failure mode may be found in the negative synergy of three

combined events: (i) the actual yield stress of the core plate steel was appreciably larger than the

expected value; (ii) improper manufacture of the welds connecting the unrestrained portion of the

core end plate and the stiffeners, with consequent failure of the welds; (iii) the inner clearance

between the yielding core and the restraining sleeve was not complied with the design value (aclearance equal to 1 mm per core side was designed, while, having detached the devices after the

test, a clearance lower than 0.5 mm per core side has been measured).

a)

-2000

-1500

-1000

-500

0

500

1000

1500

2000

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Average 1st

interstory drift (%)

BaseShear(kN)

b=1.25%

b)

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.51

stinterstory drift [%]

NormalizedbaseShear

Bare RC structure

Structure equipped withBRBs

y=0.18% b=1.25%

6.94

Fig. 3. BRB type 3: Overall response.

a)

b) c) d)

Fig. 4. BRB type 3: damage pattern.


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3.2 BRB type 4

The BRB type 4 showed the best overall performance, characterized by the complete efficiency as a

ductile fuse up to its maximum design deformation range. In fact, this system showed a symmetric

response with rounded and stable cyclic behaviour. In particular, as it can be noted in Figure 5a the

tested device showed an almost perfect symmetric response within the design interstory drift range

(1.5%). The tested structure reached a maximum interstory drift of about 3%, with a minimum

ductility capacity of about =10.5 (Figure 5b).

The experimental test was completed in conjunction with the end of the core free length working

stroke (Figures 6a, b). When the maximum displacement capacity of the tested device was achieved

two different secondary failure mechanisms were recognized: 1) local buckling and related plastic

bending of the steel plates constituting the restraining sleeve (Figures 6c and d); 2) overall brace

buckling due to the transmission of compressive forces to the sleeve when the working stroke is

exceeded (Figure 6e). In particular, among the four tested braces the latter mechanism occurred

only in one of them. Finally, at the end of the last loading cycle the tensile fracture of the inner core

was recognized in the brace that globally remained stable.

a)

-1500

-1000

-500

0

500

1000

1500

-3 -2 -1 0 1 2 3

Average 1st

interstory drift [%]

BaseShear(kN)

b)

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5 31

stinterstory drift [%]

NormalizedbaseShear

Bare RC structure

positive envelope (BRBs-type4)

negative envelope (BRBs-type 4)

y=0.18%

b=1.89%

10.5

b=2.96%

16.4

Fig. 5. BRB type 4: Overall response.

a) b) c) d) e)

Fig. 6. BRB type 4: damage pattern.

4 NUMERICAL MODELLING

The numerical modelling of the tested RC building required to take into account a number of

aspects, such as the presence of non structural elements (perimetric facing walls and partition

walls), the influence of the staircase structure and the level of damage reached in the RC members

after each performed experimental test. In particular, the damaged masonry-infilled RC structure

has been modelled by means of SAP2000. The main modelling assumptions are as follows:

1. The presence of the staircase has been neglected: in fact, after the first two experimental

pushover tests on the unbraced building, the staircase structure practically failed and it wasnot repaired.

2. Plastic hinges have been placed in their actual location, as experimentally observed.


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3. The flexural and shear capacity of the damaged internal columns have been reduced assuming

that the cross section at both column ends is composed by a central concrete core and by the

steel longitudinal rebars (the first transmitting the shear forces and the resultant of

compression axial forces, the latter able to transfer only the tensile forces);

4. The masonry infill walls have been schematized by means of equivalent diagonal struts, using

the model proposed by Al-Chaar [8].

Each brace has been modelled as a truss element characterized by the Bouc-Wen hysteretic model[9]. In particular, the model parameters (n, which is a dimensionless quantity that control theshape of the hysteretic loop, and the post-yield to elastic stiffness ratio K) have been initiallyassumed with values equal to those usually suggested for unbonded BRBs [2, 3] (i.e. n=1, whichimplies a smooth the transition from the elastic to the post-yielding regime, and K=0.025). Thishypothesis led to satisfactorily interpret the behaviour of the BRB type 4 (as shown in Figure 7),

which is the device that perfectly showed the designed performance.

0

500

1000

1500

0 2 4 6 8 10 12 14

Roof displacement (cm)

BaseShear(kN)

numerical response curve

negative envelope

positive envelope

Fig. 7. Experimental vs. numerical response of the RC structure equipped with the BRB type 4.

3 SUMMARY

A special only-steel BRB device has been designed and tested. The main peculiarity of this

innovative device consists in the possibility to hide it into the inner space between the two facades

of masonry infill walls commonly used for RC buildings. In detail, two different devices have been

tested. Both of them showed a satisfactory global response (an overall ductility of about 7 for the

first tested device and a minimum ductility of about 10.5 for the second one), even if the

performance of the first type tested was impaired by buckling of the unrestrained non-yielding

segment. In order to improve the performance of the only-steel BRB prototype, some local details

have been simplified and some geometrical proportions have been modified in such a way to

improve the robustness of the second tested device. The excellent experimental performanceconfirmed the effectiveness of the chosen technological and geometrical adjustments.


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Numerical models of the tested structure equipped with BRBs have been calibrated on the basis of

the experimental response. The BoucWen model was implemented to approximate the

macroscopic behaviour of the brace. In particular, the model parameters were determined from the

geometrical and physical properties of the brace and the set of model parameters, which are usually

suggested in the literature [2] satisfactorily predicted the inelastic behaviour of the tested systems.

It is concluded that unbonded braces represent a reliable and practical alternative to conventional

framing systems to enhance the earthquake resistance of existing structures, capable of providingboth the stiffness and the strength needed to satisfy structural drift limits. However, further

experimental investigation should be required to evaluate the cumulative ductility capacity provided

by these only-steel devices.

4 ACKNOWLEDGMENTS

The following subjects are gratefully acknowledged for having provided the financial support:

1. The RELUIS Consortium, within Project Task 5 Development of innovative approaches to

design steel and composite steel-concrete structures

2. MIUR, within PRIN 2005-2007 project Advanced steel braces with decoupled design

parameters.

3. European Commission, within PROHITECH project (Earthquake Protection of Historical

Buildings by Reversible Mixed Technologies).

REFERENCES

[1] Wada A., Nakashima M. From infancy to maturity of buckling restrained braces research.

Proceedings of the 13th World Conference on Earthquake Engineering, Vancouver (Canada)2004.

[2] Black C., Makris N., Aiken I. Component testing, stability analysis and characterization of

buckling restrained braces. PEER Report 2002/08, Pacific Earthquake Engineering ResearchCenter, University of California at Berkeley, 2002.

[3] Tsai, K.C., Lai, J.W., Hwang, Y.C., Lin, S.L. & Weng, Y.T. Research and application of

double-core buckling restrained braces in Taiwan. Proceedings of the 13th World Conferenceon Earthquake Engineering, Vancouver (Canada) 2004.

[4] Mazzolani, F.M. (Co-ord. and Ed.). Seismic upgrading of RC buildings by advanced

techniques. The ILVA-IDEM research project. POLIMETRICA Publisher, 2006.

[5] Della Corte G., DAniello M., Mazzolani F.M. Seismic Upgrading of RC buildings using

Buckling Restrained Braces: full-scale experimental tests. Proceedings of the XX C.T.A.ConferenceFirst int. Workshop on Advances in Steel Constructions, Ischia (Italy), 26-28

September, 2005.

[6] Della Corte G., Mazzolani F.M. Full-scale lateral-loading tests of a real masonry-infilled RC

building. Proceedings of the Second fib Congress, Naples (Italy) 5-8 June 2006.

[7] DAniello, M., Della Corte, G. & Mazzolani, F.M. Seismic Upgrading of RC Buildings by

Steel Eccentric Braces: Experimental Results vs Numerical Modeling. Proceedings of the

STESSA Conference, Yokohama (Japan) 14-17 August, 2006.

[8] Al-Chaar G. Evaluating Strength and Stiffness of Unreinforced Masonry Infill Structures,

US Army Corps of Engineers Engineer Research and Development Center, 2002.

[9] Wen, Y.-K. Method for random vibration of hysteretic systems. J. Eng. Mech., 102~2!,

249263, 1976.

buckling restrained braces tests eng

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